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At the Local Inspectors examination the applicant for a license may receive the problem written in some peculiar manner; but that does not matter, as all that is wanted is the date and name of the port. In any case compute all the tides for the given civil date, and in cases where a tide occurs a few minutes before the beginning of the civil date and another a few minutes after the end, they must be included, and after all the tides have been correctly computed, they should be tabulated in the order of their
Bowditch Navigator (Appendix IV) contains the high and low water "lunitidal intervals" or "corrected establishments of the ports.” Other works of the same character contain the high water interval only.
The "unitidal intervals" or "corrected establishments of the ports” are the intervals between the moon's meridian passage at a given place and the following high water and low water.
The first step in the computation of tides is the selection of the proper passage from the nautical almanac, and in doing so the astronomical date and time must be adhered to; but when the result is found, it must be converted into civil time and so expressed.
The hourly difference in the adjacent column may be given a standard value of two minutes which, when multiplied by the nearest hour of the longitude in time of the place, will give the correction to be added to the passage as taken from the almanac in west longitude and subtracted in east and gives the time of meridian passage at the port. The degree of precision required does not warrant a more exact operation for the simple reason that two or three minutes error is not a perceptible quantity.
As the Bowditch epitome is the only work containing both the
high water and low water intervals for various ports, the examiners prefer its use; but when it is not convenient to use that book, only the high water interval will be found in others and then half or quarter of a lunar day is applied to the time of high water found from the moon's meridian passage to find the times of the other tides for the given date.
A lunar day is the interval of time between two successive passages of the moon over the same meridian, which in solar time is given a value of 24h 52m, the half and quarter of which is 12h 26m and 6h 13m respectively; but as the interval of time between tides is in most cases different, it is necessary to use the low water interval from Bowditch that an accurate result may be obtained.
Beginning with the year 1916, the nautical almanac is different from those for preceding years by which nearly every examination is conducted. For that reason the almanacs for the years 1914 and 1917 are used.
EXPLANATION OF EXAMPLE 1.
On page IV of the Nautical Almanac, January 5th, the moon's meridian passage at Greenwich is 6h41m and because it is less than 12h, the astronomical date and time is 5d 06h 41m, the hourly difference is assumed to be 2m, which is sufficiently accurate for this calculation and is multiplied by the nearest hour of longitude in time of Baltimore, which gives a correction 10m, which is added because the longitude is west. The result is the moon's upper transit at Baltimore, to which is added the low water interval Oh 44m and gives the astronomical date and time of low water, which is 5d 07h 35m, and being less than 12 hours is also civil time P. M.
The lower transit is found by adding together the moon's meridian passages for the 4th and the 5th, taking care to include the days and dividing by 2, which gives the moon's lower transit at Greenwich, to which is added the correction 10m for longitude 5h west, and the result will be the moon's lower transit at Baltimore; but to be exact it is the opposite meridian. To this add the high water interval 6h 34m, which gives the time of high water 5d 01h 04m, and being less than 12 hours is also civil time P. M.
To the moon's lower transit at Baltimore add the low water interval Oh 44m, which gives the astronomical date and time of low water, 4d 19h 14m. Add 1 day and subtract 12 hours and the result will be the civil time of low water the 5th, 7h 14m A.M.
To the moon's meridian passage for the 4th, 4d 06h 00m add the correction for longitude, which will give the moon's upper transit at Baltimore, and to this add the high water interval 6h 34m. The result will be the astronomical date and time of high water 4d 12h 44m. Add 1 day and subtract 12 hours and the result will be the civil time of high water the 5th, Oh 44m A.M.
1. Example: January 5, 1914. Find the times of high waters and low waters at Baltimore, Md.
Moon's lower transit Baltimore
i High water, 5th 2 Low water, 5th 3 High water, 5th 4 Low water, 5th
oh 44m A. M.
14 A. M. I 04 P. M. 7 35 P. M.